Our long-term objective is to contribute to the understanding of the regulation of hemoglobin synthesis and erythroid differentiation in normal and abnormal hematological conditions, with particular emphasis on the roles of heme and protein synthesis in these processes. Protein synthesis in intact reticulocytes and their lysates is dependent on the availability of heme. Heme serves as the prosthetic group of the hemoglobin, the predominant protein in red blood cell (RBC) and its late precursor cells. Biochemical studies have shown that, under conditions of heme-deficiency or iron-deficiency, protein synthesis is inhibited at the level of initiation due to the activation of the heme-regulated translational inhibitor (HRI). HRI is a heme-regulated protein kinase that phosphorylates the a-subunit of eukaryotic initiation factor 2 (eIF2a). Phosphorylation of the a-subunit of eIF2 impairs its recycling in translational initiation and results in the cessation of protein synthesis. The research designs are (1) To Elucidate the Molecular Mechanism by which HRI Regulates Red Blood Cell Production, (2) To Assess the Protective Role of HRI in Red Blood Cell Disorders, and (3) To Delineate the Molecular Mechanism of Stress-Activation of HRI. The methods to be employed are the generation of genetically modified mice, hematological analysis, histological and electron microscopic examinations, colony culture assays for erythroid progenitors, Western-blot analysis and protein kinase assays, cell culture, phenotypic examination of mouse embryos, recombinant DNA techniques. Our recent study indicates that HRI is not only necessary for globin synthesis, but also for heme biosynthesis. HRI is also important for the survival of mice with thalassemia and erythropoietic protoporphria. Our proposed study may lead to potential application of HRI in treating human patients with red cell diseases.